With consumer printer market growth, inkjet printing has become a broadly applicable technology for supplying small quantities of liquid to a surface in an image-wise way. Both drop-on-demand and continuous drop devices have been conceived and built. Whilst the primary development of inkjet printing has been for graphics using aqueous based systems with some applications of solvent based systems, the underlying technology is being applied much more broadly.
There is a general trend of formulation of inkjet inks toward pigment based ink. This generates several issues that require resolution. Further, for industrial printing technologies, i.e. employing printing as a means of manufacture, the liquid formulation may contain solid or dispersed components that are inherently difficult to handle with inkjet processes.
A new continuous inkjet device based on a MEMs formed set of nozzles has been recently developed (see U.S. Pat. No. 6,554,410). In this device a liquid ink jet is formed from a pressurized nozzle. One or more heaters are associated with each nozzle to provide a thermal perturbation to the jet. This perturbation is sufficient to initiate break-up of the jet into regular droplets through the well known Rayleigh-Plateau instability. By changing the timing of electrical pulses applied to the heater large or small drops can be formed and subsequently separated into printing and non-printing drops via a gaseous cross flow.
Inkjet drop generation devices are microfluidic devices in that they employ very small scale liquid channels. The implication of this is that the Reynolds number
  Re  =            ρ      ⁢                          ⁢      UL        μ  where ρis the liquid density (kg/m3), U is a characteristic velocity (m/s), L a characteristic length (m) and μ the liquid viscosity, (Pa·s), is sufficiently small that inertial effects are small and the flow is predominantly laminar in nature. For a typical continuous inkjet system the velocity might be 20 m/s and a length might be 5 μm with a density approximately 1000 kg/m3 and a viscosity of 1 mPas. The Reynolds number is therefore approximately 100. The transition to turbulent flow in a straight pipe occurs at Re above approx 2000.
Microfluidic devices where the liquid flow is laminar necessarily prevent mixing. In fact the only mechanism available for mixing is diffusional flow. For example, consider a T junction in which two fluids are injected to flow alongside each other. How far down the channel must the fluids flow before the channel is homogenized? A simple estimate requires the particles or molecules to diffuse across the entire channel, giving a time tD˜w2/D, where w is the width of the channel and D is the diffusion constant. During this time, the material will have moved a distance z˜U0w2/D down the channel, so that the number of channel widths required for complete mixing would be of order
      Z    w    ≈                    U        0            ⁢      w        D    ≡  Pe
The dimensionless number on the right is known as the Péclet number (Pe), which expresses the relative importance of convection to diffusion. In this example, the number of channel widths required for full mixing varies linearly with Pe. Using the diffusivities in the table below estimated using the Stokes-Einstein relation, we see that even a dye molecule flowing with the fluid through a 10 μm channel at 1 m/s requires Pe˜250000 channel widths to completely mix. Alternatively, that dye molecule flowing with the fluid at 1 m/s would require a pipe length z˜25 mm to diffuse 1 μm.
Characteristic Diffusivities in water at room temperatureTypicalDiffusionParticlesizeconstantSolute ion10−1nm2 × 103μm2/sDye molecule5nm40μm2/sColloidal particle100nm2μm2/sBacterium1μm0.2μm2/sMammalian/human cell10μm0.02μm2/s
When a liquid flows across a surface the velocity of the liquid at the solid surface is zero. In a long pipe the maximum liquid velocity is found in the centre of the pipe and the velocity profile across the pipe is parabolic. This is referred to as Poiseiulle flow. However, on entry to a pipe there is a finite distance, the entry region, where the flow field adopts that consistent with the pipe geometry. In the terminology of fluid mechanics there is a boundary layer that forms and grows until it is the size of the pipe at which point fully developed flow is achieved. The boundary layer thickness may be calculated as
  δ  =                    μ        ⁢                                  ⁢        x                    ρ        ⁢                                  ⁢        U            
where δ is the boundary layer thickness (m), μ is the liquid viscosity (Pa·s), x is the distance from the start of the pipe (m), ρ is the liquid density (kg/m3) and U the liquid velocity (m/s). The nozzle in an inkjet droplet generator is a very short pipe i.e. too short for fully developed flow to be achieved. Therefore only a boundary layer thickness of liquid next to the nozzle wall is sheared.
There are numerous known methods and devices relating to the formation of droplets.
EP1364718 discloses a method of generating encapsulated droplets via co flowing immiscible liquids. In this method the liquids are supplied by coaxially arranged nozzles, which are difficult to manufacture as an array. Further, this method relies on a strong electrostatic field to ensure break-up of the coaxially arranged liquids.
JP1996207318 again uses coaxial tubes and electrostatics to break off a droplet. The centre tube in this case can supply colloidal particles or a plurality of them to provide a colour level. Electrophoretic means can stop the flow of particles by arrangement of electric fields.
U.S. Pat. No. 6,713,389 describes placing multiple discrete components on a surface for the purpose of creating electronic devices.
U.S. Pat. No. 5,113,198 describes using a carrier gas stream to direct vaporous dyes toward a surface. This uses co flowing gas streams but no liquids.
U.S. Pat. No. 6,377,387 describes various methods for generating encapsulated dispersions of particles.
WO2006/038979 describes a drop on demand piezo electric device where liquids are brought together external to the device structure.